The present invention pertains to a process and a device for illustrating a printing form for wet offset printing. The present invention is used especially in newspaper offset printing and preferably in web-fed newspaper offset printing.
In newspaper offset printing, the preliminary printing stage has changed greatly as a consequence of the progressive development of computer technology. Operations that were previously carried out manually have been replaced by so-called computer-to technologies. The current stage of development has been reached with computer-to-plate in newspaper printing. The further development is toward computer-to-press, i.e., toward direct illustration in the press.
A printing form cylinder that can be illustrated directly has been known from U.S. Pat. No. 5,293,817. The printing form cylinder has a porous outer jacket. The moistening is performed through the interior of the cylinder through the porous outer jacket. The porosity of the cylinder jacket is between 20% and 45%. The diameter of the pores of the cylinder jacket decreases toward the outside of the cylinder jacket and is between 3 xcexcm and 100 xcexcm. The pores of the cylinder jacket communicate with one another. The illustration is performed according to a thermo transfer or ink jet process by means of an image information transfer means. As an alternative, the use of a heated electrode in the form of a pin in order to apply oleophilic material to the cylinder jacket is mentioned.
The object of the present invention is to make possible the illustrating of a printing form in a wet offset printing press in a simple, inexpensive and accurate manner.
According to the invention, a process for illustrating a printing form for wet offset printing is provided, in which a printing style is produced on an ink-transferring surface of the said printing form by forming ink-absorbing and ink-repelling areas. For illustration, the steps are as follows:
a) the ink-transferring surface is wetted with a moistening agent,
b) the areas of the moistened surface that are to be made into ink-absorbing areas, are dried in a specific manner, and
c) a material that absorbs printing ink is uniformly applied to the ink-transferring surface with the areas still dry.
According to another aspect of the invention, a process for illustrating a printing form for wet offset printing is provided in which a printing style is produced on an ink-transferring surface of the said printing form by forming ink-absorbing and ink-repelling areas. For illustration, the steps are as follows:
a) a flowable material that absorbs printing ink is uniformly applied to the ink-transferring surface,
b) the material is cured in a specific manner on the areas that are to be made into ink-absorbing areas, and
c) it is removed from the areas that are to be made into ink-repelling areas by means of moistening agent.
According to another aspect of the invention, a device for illustrating a printing form of a rotary printing press for wet offset printing is provided. The device includes a printing form cylinder with the printing form a moistening means for wetting an ink-transferring surface of the said printing form with moistening agent, and an application and image transfer means, with which a printing style with ink-absorbing areas and moistening agent-absorbing areas is produced on the ink-transferring surface by applying a material absorbing printing ink. The application and image transfer means comprises an application means for the uniform application of the material absorbing printing ink over the ink-transferring surface and an image transfer means. The printing style is produced by the uniform application of printing ink-absorbing material by means of the application means and by specific irradiation of the areas of the ink-transferring surface that are to be made into ink-absorbing areas by means of the irradiation means.
The present invention is based on a process for illustrating a printing form for wet offset printing, in which a printing style is produced on a non-illustrated surface of the printing form by preparing ink-absorbing and ink-repelling areas. The surface will hereinafter be called the ink-transferring surface in both the illustrated and non-illustrated state because of its ink transfer function.
According to the present invention, the printing style, i.e., the illustration, is produced by wetting the yet non-illustrated ink-transferring surface with a moistening agent, drying the areas of the moistened surface that are intended to absorb ink in a specific manner, i.e., in the pattern of an image, and subsequently applying material that absorbs printing ink and preferably repels moistening agent to the ink-transferring surface with the still dry areas. Like printing ink itself, the material is repelled by the moistening agent, i.e., it is not transferred to the moist areas. Since the supply of moistening agent preferably takes place uniformly and continuously during the illustration as well, the material is preferably applied as quickly as possible after the drying of the ink-transferring surface, which drying is performed in the pattern of the image, so that the dried areas are not wetted again with moistening agent prior to the application of the ink-absorbing material.
The thickness of a film of moistening agent wetting the ink-transferring surface should be at most 1 xcexcm, averaged over the area of one image pixel, in order to keep the energy of evaporation low. The thickness is preferably set at a value between 0.2 xcexcm and 0.5 xcexcm.
Without leaving the scope of the present invention, the process may be modified such that for illustration, a flowable material is first applied uniformly to the ink-transferring surface, which absorbs printing ink and preferably repels moistening agent, after which this material is specifically cured or is brought to a curing in the areas that are to be made into ink-absorbing areas, and it is removed by means of moistening agent in the areas that are to be made into ink-repelling areas, where it is still flowable.
The said two alternatives of the process have the advantage that the ink-transferring surface is first wetted or covered uniformly, for which moistening water is used in one case and ink-absorbing and moistening agent-repelling material in the other, and the uniformly wetted or covered surface of the printing form is then specifically provided with the printing style by drying the moistening agent or curing, especially by drying in the ink-absorbing/moistening agent-repelling material, where the dried areas or the areas with the cured material form the ink-absorbing areas in the printing style. An applicator, with which an ink-absorbing and moistening agent-repelling material is directly applied in the pattern of an image, is not needed for carrying out the process. By performing only a uniform application of material to produce the printing style in both alternatives of the process, the cost for the applicator for the illustration is markedly reduced.
Uniform application is defined according to the present invention as any application of material for illustration which is not performed in the pattern of an image itself. The material is preferably applied over the entire surface of the printing form or at least in strips of the surface.
The ink-absorbing/moistening agent-repelling material is preferably printing ink, especially printing ink of the current production, in which the illustrated printing form will then be used.
A special advantage of both alternatives of the process according to the present invention is that an ink applicator, e.g., an inking roller, which is present for the current production anyway, can be used to apply the ink-absorbing and preferably moistening agent-repelling material in the course of the illustration. The use of such an ink applicator, especially an inking roller, also corresponds to an especially preferred exemplary embodiment of the present invention. However, it would also be possible to provide a separate applicator roller or another, suitable applicator, e.g., a spray means, for the illustration only or for the illustration and for the subsequent application of the ink. However, the material should not be applied in a controlled manner according to the pattern of an image in the case of such an embodiment, either.
Only the drying of the moistening agent and/or the curing of the ink-absorbing/preferably moistening agent-repelling material is always performed in the pattern of an image. An image transfer means preferably used for this purpose is formed with semiconductor lasers, especially lasers, preferably laser diodes, especially preferably by an array or a plurality of arrays of infrared laser diodes.
The treatment of a material or moistening agent previously applied uniformly to the ink-transferring surface of the printing form, which is performed immaterially and in the pattern of an image according to the present invention, also makes it possible to produce the image more accurately than this is possible by the direct application of material in the pattern of an image.
Furthermore, the present invention pertains to a device for illustrating a printing style in a rotary printing press for wet offset printing. The press is preferably a web-fed rotary printing press for newspaper offset printing. The device comprises a printing form cylinder with the printing form, a moistening means for wetting an ink-transferring surface of the printing form with moistening agent and an application and image transfer means, with which application and image transfer means a printing style with ink-absorbing areas and moistening agent-repelling areas is produced on the ink-transfer surface by applying a material that absorbs printing ink and repels moistening agent.
According to the present invention, the application and image transfer means has an application means for uniformly applying a flowable material that absorbs printing ink and preferably repels moistening agent, and an irradiation means, preferably an exposure device. The printing style is produced by a combination of material application by means of the application means and irradiation of the areas of the ink-transferring surface that are to be made into ink-absorbing areas by means of the irradiation means. The application means is preferably an application means that transfers the printing ink to the printing form cylinder in the current production.
In a large rotary printing press with a plurality of printing form cylinders, such an application and image transfer means is preferably associated with each of these printing form cylinders. The cost advantages of the direct illustration according to the present invention increase with increasing press size, i.e., increasing number of printing form cylinders, especially if the print production is frequently to be changed and down times are to be minimized.
Furthermore, the present invention pertains to a printing form that is permeable to moistening agent and to a process for preparing such a printing form. The printing form is a printing form for a wet offset rotary printing press. The printing form has a surface that can be or is illustrated for transferring printing ink.
The non-illustrated printing form is moistening agent-friendly or it absorbs moistening agent and is preferably hydrophilic at least on an ink-transferring surface. Like prior-art printing forms as well, the printing form may be, e.g., a printing form plate or preferably a printing form shell, which is fastened to a carrier cylinder, e.g., by means of a prior-art clamping device. Such inherently stable printing forms, which can be or are illustrated, are as such also the subject of the present invention. The carrier cylinder forms the printing form cylinder together with the fastened printing form in this case. The printing form may, in principle, also be a cylinder sleeve. However, the drawback of such a printing form sleeve would be that the carrier cylinder would be able to be mounted rotatably on one side only to make it possible to replace the printing form sleeve in a simple manner. Printing form cylinders that have a printing form that can be or already is illustrated on a cylinder jacket surface, in which case the printing form cannot be removed, are also the subject of the present invention; at any rate, the printing form cannot be removed without destruction in this embodiment.
The printing form is permeable to a moistening agent in a radial direction. In the case of a printing plate that is fastened to a carrier cylinder, the direction is indicated relative to the mounted state. The printing form cylinder including the printing form has a means by means of which the moistening agent can be fed to the printing form. The carrier cylinder is defined according to the present invention as the cylinder body of the printing form cylinder on which the printing form is arranged, either as an independent printing plate or, as was described above, as a fixed part. The moistening agent, especially moistening water, is brought from the rear side of the printing form to the ink-transferring surface of the printing form in both embodiments. Since passage channels for the moistening agent open onto the ink-transferring surface because of the radial permeability of the printing form, the moistening of the printing form, i.e., of the ink-transferring surface, and consequently the ink absorption or repulsion can be brought about by specifically closing passage channels on the ink-transferring surface. No moistening agent can reach the ink-transferring surface in the area of closed passage channels, so that ink is absorbed in the area enclosing the passage channel.
Even though the ink-transferring surface can be prepared, in principle, by perforating an initially closed surface, the printing form preferably has an outer material layer that is porous as the printing layer.
The printing form is built up layer by layer according to the present invention and has an outer printing layer with the surface that can be or is illustrated and an adjacent, subjacent sublayer. The flowability (the reciprocal of the flow resistance) through the printing form preferably decreases abruptly several fold according to the present invention at a boundary layer from the sublayer into the printing layer. The resistance to flow increases correspondingly. These two layers may advantageously consist of different materials and they can also be optimally adapted to different functions as a result according to the present invention as well.
Flowability is defined according to the present invention as the volume of the moistening agent used that flows through a layer of a given thickness relative to a pressure difference acting over this layer per unit of time and area, where the area is defined as the outer surface of the layer. The flowability as a material characteristic will hereinafter be related to the non-illustrated state of the ink-transferring surface.
An abrupt reduction according to the present invention is defined not only as a sudden change in flowability, which change can be considered to be discontinuous for practical purposes, but also as a continuous change. In the latter case, the flowability has a steep gradient at the transition from the sublayer into the printing layer. A transition zone from the sublayer into the printing layer, which can never be avoided altogether in practice, and in which the change in flowability according to the present invention takes place, is always thinner than the printing layer. The steepest possible reduction in flowability (an increase in flow resistance) from the sublayer into the printing layer is desirable according to the present invention.
According to a process for preparing such a cylinder, the sublayer is first formed, preferably by a mat made of rustproof metal fibers. Such a mat advantageously has high tensile strength and compressive strength compared with materials of equal porosity. Sintered mats of rustproof metal fibers, rolled to a defined thickness, are especially suitable. Suitable mats have been known from the filter technology applications.
The printing layer is formed by coating the sublayer, preferably by plasma spraying. It is preferably formed by a ceramic layer.
The flow resistance through the sublayer is several times lower than that through the printing layer directly adjacent to it in both the radial direction and the axial and circumferential directions. The flow resistance through it is preferably at least a hundred times and especially at least a thousand times lower than the flow through the printing layer. The sublayer is preferably sealed against the passage of moistening agent at its free edges. The entire printing form is preferably sealed at its free edges.
The flowability through the sublayer is several times higher than that through the printing layer directly adjacent to it in both the radial direction and the axial and circumferential directions. The flowability through it is preferably at least a hundred times and especially at least a thousand times greater than the flowability through the printing layer. The sublayer is preferably sealed against the passage of moistening agent at its free edges. The entire printing form is preferably sealed at its free edges.
Due to the layered structure of the printing form according to the present invention, the resistance to flow or the flow through the printing form as a whole is determined, in a practical approximation, exclusively by the printing layer. The printing layer preferably has a uniform material texture and, at least partly as a result of this, it can be prepared such that it is optimally adapted to the required print fineness. Its structure and texture is such that it is traversed by capillary pores, which are very fine and have a high density by surface on the ink-transferring surface. At least one such capillary pore opens onto the surface per image pixel. The porosity of the printing layer is kept at a low level at the same time. It is preferably below 20%. The porosity is open porosity.
The printing form has the additional advantage that an especially well-defined pressure drop through the layer can be set by means of a single, thin layer, the printing layer, which is uniform in itself With respect to the guiding of moistening agent, the subjacent sublayer has the task of distributing the moistening agent uniformly over the area under the printing layer. The moistening agent level, which is one of the factors determining the overpressure on the rear side of the outer printing layer, is built up in this sublayer. A type of moistening agent lake is formed on the rear side of the outer printing layer due to the design according to the present invention. Thus, the moistening agent presses the printing layer in a particularly uniform manner, so that defined pressure conditions become established on the whole and accurate guiding of moistening agent is possible. Furthermore, the moistening agent pressure is adapted to the particularly required rates of feed of moistening agent to the surface practically without a time lag during acceleration and deceleration phases of the printing form cylinder, e.g., during the speed-up or slowdown of the press.
The printing form is used especially preferably in combination with the illustration process according to the present invention and/or in a device according to the present invention. However, it is not limited hereto, but it may also be used advantageously in combination with prior-art processes and devices for illustrating internally moistened printing forms.
The passage channels of the printing layer, which are preferably the above-mentioned capillary pores, preferably have a mean diameter of 0.1 xcexcm to 5 xcexcm, especially measured at the points at which they open onto the ink-transferring surface. The porous printing layer preferably has an average peak-to-valley height Ra in the range of 0.2 xcexcm to 5 xcexcm and preferably an average surface roughness Rz in the range of 0.2 xcexcm to 10 xcexcm on the ink-transferring surface.
The sublayer is traversed by passage channels, e.g., connected pores, which have a diameter of 10 xcexcm to 2 mm and preferably 10-50 xcexcm. The diameter is defined as the diameter of a circle that has the mean cross-sectional area of the passage channels of the layer in question. If it is formed by a mat, the laminar diameter, determined according to ASTM F 902, is the suitable parameter for the characterization. The laminar diameter should be between 10 xcexcm and 100 xcexcm.
The thickness of the printing layer in the radial direction is preferably between 50 xcexcm and 500 xcexcm, and the thickness of the sublayer is preferably between 500 xcexcm and 3 mm.
The printing layer preferably has a high absorption coefficient for infrared radiation. The absorption coefficient should be at least 0.9.
Since the moistening agent is preferably evaporated by infrared radiation in the first alternative of the process and there is only a slight absorption of infrared laser radiation, which is preferably used, in the moistening agent film in the near infrared when moistening water is used as the moistening agent, heating and evaporation of a moistening agent take place indirectly by the heating of the printing layer. To achieve an intense local heating of the printing layer, the material selected for the printing layer is preferably a material with a thermal capacity that is lower than the thermal capacity of the moistening agent. The thermal capacity of the printing layer is especially preferably less than 1 J/g. Furthermore, the printing layer is designed such that the thermal conductivity of this layer is markedly lower than the thermal conductivity of the moistening agent. The thermal conductivity is preferably less than 0.2 W/(mxc2x7K).
Preferred materials for the printing layer are dark, ceramic materials, e.g., an Al2O3xe2x80x94TiO2 mixture.
An independent printing form is preferably composed of at least three layers with a printing form carrier, which lets moistening agent through, the sublayer applied thereto, and the printing layer applied to the sublayer. The printing form carrier is preferably made of a metallic material. It may be designed as a flat plate that can be arched or as a preformed shell, especially as a rigid cylindrical half shell. A printing form applied to the carrier cylinder may also have such a multilayer structure.
A perforated printing form carrier has holes with a diameter preferably in the range of 0.5 mm to 5 mm or openings of an equal area, which are arranged in the entire area of the printing form spaced preferably at 5 mm to 50 mm from one another. However, the hole density can be considerably reduced by enlarging the areas per hole and/or by forming a channel structure on the outer surface of the printing form carrier.
The sublayer is applied to the printing form carrier; in particular, it is fastened to it as a whole, preferably glued or bonded by means of a heat-resistant adhesive.
The dry areas on the ink-transferring surface of a printing form that is permeable to moistening agent are preferably obtained by specifically closing the passage channels opening onto the ink-transferring surface.
The amount of moistening agent being discharged on the ink-transferring surface per unit of time is regulated by setting the moistening agent pressure, advantageously by setting the amount of moistening agent in the printing form cylinder. The moistening agent pressure is increased because of the centrifugal forces by increasing the amount of moistening agent on the rear side of the printing form. Furthermore, the rate of feed of the moistening agent to the printing form cylinder is increased and decreased in proportion to the speed of printing.
The amount of moistening agent being discharged on the ink-transferring surface per unit of time, i.e., the rate of flow through the printing form, depends, in an approximation that is fully sufficient for practice, only on the flow through the outer printing layer. The pressure difference over the outer printing layer increases at constant speed of rotation approximately linearly with the moistening agent level that becomes established on the rear side of the printing layer. The flowability through the printing layer can be set only by selecting the thickness of the printing layer, because the printing layer has everywhere an essentially constant porosity and capillary pore density. The sublayer is also homogeneous in this sense. An especially accurate metering of the amount of moistening agent being discharged on the ink-transferring surface can be performed due to the splitting of the function of the uniform distribution of the moistening agent and the setting of the flowability through the printing form. The thickness of the moistening agent film on the surface can be set very accurately, especially at a very low value. At constant cylinder speed, exactly as much moistening agent is fed to the rear side of the printing form as is to be discharged on the ink-transferring surface. The level of equilibrium of the moistening agent level in the sublayer on the rear side of the printing layer will then become established by itself as a function of the speed of rotation of the printing form cylinder. The setting also takes place without a time delay because of the design of the printing form according to the present invention in the case of a change in the speed of rotation.
The overpressure on the rear side of the printing layer shall not exceed 100 mbar. The moistening agent level on the rear side of the printing layer should not exceed the thickness of the sublayer at least at the equilibrium between the intake and the discharge. The thickness of the sublayer and the flowability through the printing layer are correspondingly preferably coordinated with one another.
A preferred possibility of closing is obtained by heat-induced, preferably laser-induced, image-dependent toning. The wet offset printing is known to be based on the repulsion of ink by moistening agent on the moistened areas of the printing form. If the feed of moistening agent is insufficient, ink will also be absorbed in the non-image areas. This operation is generally called toning.
The ink-transferring surface of the printing form is moistened from the inside in this first alternative of the process and it is then dried in an image-dependent manner by means of the image transfer means, preferably by means of infrared lasers. The image areas are dried in this process and are inked immediately thereafter. During inking, the ink is transferred onto the dried areas, while the moist areas remain ink-free. The ink clogs the passage channels in the dried areas, so that no more moistening agent can reach the ink-transferring surface in these areas.
In the second alternative of the process, the printing form is heated in an image-dependent manner by means of the image transfer means after the inking. The ink dries in the heated areas and consequently also in the passage channels and at the mouths of the passage channels. The ink having dried in the passage channels can no longer be displaced during the subsequent supply of moistening agent, while the flowable ink is displaced by the moistening agent. The moistening agent pressure may be slightly increased compared with the moistening agent pressure in the production until the free running of the printing form.
The illustration of the printing form, i.e., the ink having dried in the passage channels, can be removed by a prior-art printing form-washing means and/or by the inner moistening with a moistening agent pressure that is increased compared with that prevailing during the production.
As an alternative to the above-mentioned closing by means of ink, a monomer or a mixture of monomers may also be applied, especially sprayed, onto the ink-transferring surface of the printing form in a subvariant of the second alternative of the process. Polymerization is induced by means of the image transfer means. The plastic formed in the process closes the passage channels, e.g., by forming polystyrene from styrene under the effect of heat. The closed pores can be freed again by repeated heating by decomposing the polymer and removing it by supplying moistening agent. The moistening agent is fed in preferably from the inside during the erasing of the image. However, washing from the outside is also possible and it especially supports this process.
In an exposure process for illustrating the printing form, the printing form and an image transfer means perform a relative movement, whose direction and velocity are preset, and during which the surface of the printing form to be illustrated is exposed pixel by pixel in the pattern of an image. As in prior-art printing forms, the pixels of the printing style to be produced are also arranged in columns and lines at right angles thereto on the ink-transferring surface of the printing form. Consequently, a pixel column extends in the direction of the column and a pixel line extends in a line direction that is at right angles thereto. The above-mentioned relative movement takes place either in the direction of the column or in the direction of the line. If the printing form is the printing form of a printing form cylinder in a rotary printing press, the direction of the column is the direction of printing and the direction of the line is the longitudinal direction of the cylinder. In the case of illustration in the press, the direction of the relative movement, to which the width and length data of the light-emitting areas are related, is the direction of printing. However, illustration of a printing form outside a press is also conceivable, in which case illustration can also be performed in the direction of the line as the standard direction of the relative movement.
The strip-like laser spot may advantageously also be used for the especially fine setting of the surface coverage and consequently the tonality steps without changing the geometry of an optical imaging means for focusing the laser light on the ink-transferring surface. By changing, especially reducing, the on time of the semiconductor lasers, pixels of any extension in the direction of the relative movement can be produced. In particular, it is possible to produce pixels that have very small areas, but are not square, but strip-shaped. The number of gray scales, which can be represented, is known to be obtained from the screen width and the pixel area. As a general rule, the screen cell width is equal to the reciprocal value of the screen width. The area of the screen cell is calculated from this by squaring. The maximum number of tonalities that can be represented is obtained by dividing the result by the pixel area. Thus, a screen cell has an area of 250xc3x97250 xcexcm2 in the case of, e.g., a 40 screen. A maximum of 16 gray scales can be represented in the case of a pixel width of 62 xcexcm in case of square pixels. If rectangular pixels of 62 xcexcmxc3x9731 xcexcm are produced, there already are 32 gray scales. The illustration time is the same in both cases. The finer graduation of the gray scales makes it possible to better compensate nonlinearities occurring in the print, such as increase and decreases in tonality, than it is possible with laser spots, which have a very short extension according to the present invention in the direction of the relative movement. If the printing form is arranged on a printing form cylinder, an angle transmitter for the printing form cylinder or for an associated rubber blanket cylinder has a correspondingly higher resolution for this compensation, or intermediate increments are formed with an electronic unit by interpolation.
The image transfer means preferably comprises an array or a plurality of arrays of pulsed or current-pumped semiconductor lasers, especially pulsed infrared lasers, especially preferably laser diodes. Narrow, strip-like, preferably rectangular laser spots are produced by means of the lasers on the surface to be illustrated, and the width of these laser spots, measured in the direction of the relative movement on the surface to be illustrated, is several times smaller than a width of an image pixel of the surface to be illustrated, which latter width is measured in the same direction, and it is also several times smaller than a laser spot length measured at right angles to the laser spot width. A pulse time per laser spot is now several times longer than a period during which a section that corresponds to the laser spot width is covered during the relative movement between the printing form and the laser array.
In a preferred embodiment, the illustration is performed in the press directly on the printing form cylinder. The relative movement between the printing form and the laser array is brought about in this case by a vertical movement by rotating the printing form cylinder and a horizontal movement directed at right angles hereto by shifting the laser array. The image information is transferred column by column to the printing form during the rotation of the printing form cylinder. Columns located next to one another are illustrated one after another due to the horizontal shift of the laser array until the entire image has been recorded. The laser spots produced on the ink-transferring surface of the printing form by means of the lasers now have a width measured in the vertical direction and a length measured in the horizontal direction. The horizontal movement of the laser array during the illustration preferably takes place continuously with the printing form cylinder rotating. Optimally short illustration times can thus be obtained.
The resolution of the exposure device depends on the section by which the laser array is shifted during one revolution of the cylinder. This section is preferably between 84 xcexcm and 28 xcexcm. This corresponds to a preferred resolution of 300 to 900 dpi. The laser spot length is preferably 30 xcexcm to 90 xcexcm and the laser spot width is preferably 1 xcexcm to 10 xcexcm. The laser pulse time is between 1 xcexcsec and 50 xcexcsec, depending on the resolution of the device, the output of the semiconductor laser, and the speed of illustration. The illustration is performed by the axial shift of the array, of which there is at least one, along the printing form cylinder, which is rotating during the illustration. The overall laser array may be formed by replaceable modules with, e.g., 64 diodes per printing zone.
The ratio of the laser spot length to the laser spot width is preferably at least 10:1 and especially preferably at least 20:1.
One advantage of the image transfer means according to the present invention is that a mechanically accurate adjustment is not necessary. Software adjustment is preferably performed.
The image transfer means preferably comprises an array or a plurality of arrays of, e.g., 256 semiconductor lasers in 4 arrays with 64 lasers each for one printing zone.
Semiconductor lasers with narrow, strip-like, preferably rectangular light-emitting areas, which are characteristic especially of infrared laser diodes, are used to produce a narrow, strip-like laser spot. The emitted laser light is focused on the ink-transferring surface by means of an optical imaging means. The laser output can be kept at a low level due to the preferred elongated strip shape of the laser spot, the use of a simple optical system, which is made possible hereby, in conjunction with the correspondingly narrow strip shape of the light-emitting areas and the long pulse time per image pixel. The energy consumption is also lower despite the longer pulse time than in the case of a laser spot produced directly or approximately in the pattern of the image pixel of the printing style to be produced on the ink-transferring surface.
The image transfer means correspondingly comprises pulsed or current-pumped semiconductor lasers, especially infrared lasers, especially preferably infrared laser diodes, with light-emitting, strip-like areas, whose width, measured in the direction of the relative movement, is several times smaller than a width of an image pixel on the surface to be illustrated, which latter width is measured in the same direction, and it is also several times smaller than a length of the light-emitting area.
The lasers preferably emit in the infrared or visible range, especially in the wavelength range of 700-1,400 nm.
An optical imaging means, preferably with at least one lens per light-emitting area, is provided for the light-emitting areas. The lenses of the imaging means are preferably arranged in the form of one or more arrays, advantageously as one or more lens arrays made of plastic, which can be mass-produced at low cost and are easy to mount. The lasers are fastened in or on a housing in such an orientation in relation to one another that their light-emitting areas have parallel longitudinal directions. Finally, the image transfer means comprises a triggering electronic unit, with which the lasers are triggered such that one laser pulse time is several times longer than the period during which a section that corresponds to the width of the light-emitting area is covered during the relative movement.
The exposure device does not need a fiber output. This also makes the device inexpensive and increases its efficiency. A laser carrier of the image transfer means is preferably water-cooled.
A triggering electronic unit for the image transfer means preferably comprises sufficient storage capacity for two bit maps, namely, one bit map for a current image and the second bit map, of which there is at least one, for a next image. Furthermore, the triggering electronic unit comprises a power electronic unit for each of the lasers. The triggering electronic unit is coupled with a position transmitter of the printing form cylinder, from which it receives the rotation angle position of the printing form cylinder and the position of the array, preferably of each array module, in order to synchronize the laser pulses with the movement of the printing form cylinder. The triggering electronic unit is coupled with a server PC for data transmission.
While the current image is being printed, the new image can be loaded into the memory of the triggering electronic unit. The new image is thus available for the next production within the triggering electronic unit during the current production. After the illustration of the printing form, but at the latest after the end of the current production, the memory of the old image is made free and it can take over the data for the production after next. The memory of the new image becomes the memory of the current image for the next production and the memory of the image that was previously the current image becomes the memory for the new image. The triggering electronic unit is preferably an integral part of the image transfer means directly at the site of the laser array; it is preferably moved along.
At the level of the press, the server computer or a plurality of server computers receives/receive the separated image data, which have been converted into the half-tone dots, in the form of bit maps. One or more printing couples of the press are associated with each server. The data transmission to the triggering electronic unit takes place via a high-speed local network.
The image transfer means, especially its arrangement in the printing press, the design of one or more laser arrays, the assignment and the mode of action of the triggering electronic unit, as well as the division of the tasks between the server and the triggering electronic unit can be advantageously used universally and are not bound to the conduct of the illustration process specified in the claims or to the device used for this purpose, even though the image transfer means is preferably used in this illustration and especially preferably in combination with this illustration and the above-described printing form.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.